1. Field of the Invention:
This invention relates to cold reduced, low carbon, low alloy steel strip and sheet having high yield strength in combination with ductility higher than that previously attainable and to a method for production thereof. More specifically, the present invention provides cold rolled steel strip and sheet stock having a 0.2% yield strength of at least 90 ksi with an elongation in 2 inches of at least 10%, or a cold rolled strip and sheet stock having a 0.2% yield strength of 45 to 65 ksi with an elongation in 2 inches of at least 25%, the composition for each embodiment being substantially the same. The invention further relates to a metallic coated product having a steel substrate exhibiting the above properties.
2. Description of the Prior Art:
High strength cold rolled steel has generally been produced previously by either of two approaches. One approach is to make relatively large additions of strenghening elements such as manganese (greater than 1%) and silicon (greater than 0.3%) to a steel containing more than 0.1% carbon, together with lesser additions of other strenghening alloying elements such as titanium, columbium, zirconium, and vanadium. Annealing of such steel produces high yield strengths by precipitation hardening.
Another approach is to produce a high strength steel containing carbon and nitrogen (together with small amounts of strengthening alloying elements) and subject the steel to special annealing treatments which results in an only partially recrystallized microstructure.
In both the above approaches, high strength is achieved only at the sacrifice of ductility and formability.
U.S. Pat. 3,761,324, issued Sept. 25, 1973, to J. A. Elias and R. E. Hook, disclose hot rolled and cold rolled strip and sheet material having a wide range of mechanical properties. In this low carbon steel (maximum carbon content 0.015%), columbium is added in excess of the amount required to combine with all the carbon and free nitrogen, so that uncombined columbium is present. This patent contains a recognition that columbium retards the recrystallization rate, thereby making possible the production of high strength hot-dip metallic coated products. However, at the maximum yield strength of 90 ksi for the steel of that invention, the elongation is less than 10%.
U.S. Pat. 3,671,334, issued June 20, 1972, to J. H. Bucher et al, discloses a renitrogenized columbium-bearing steel, and cold rolled, strain-aged articles formed therefrom having a yield strength of 70 to 90 ksi. The process of this patent involves a cold reduction of at least 50%, annealing to restore ductility with a consequent reduction in yield strength to about 50 to 55 ksi, pre-straining and heat treating to obtain 70 to 90 ksi yield strength by precipitation hardening. Forming into articles follows the anneal to restore ductility and precedes the precipitation hardening heat treatment. Percent elongation values of about 20% maximum were obtained at a yield strength of about 70 ksi.
It is evident from the above background of the prior art that there is not now available a low carbon steel which can be cold reduced to obtain high yield strength and retain sufficient ductility to permit forming into articles of final use without subsequent strain-aging and precipitation hardening.